1. Field of the Invention
The present invention relates to positioning mechanisms, particularly to an apparatus for positioning receptacles which contain analyte samples to be assayed by a measuring instrument, such as a photometer.
2. Description of Related Art
At the present time, various instruments are used to measure photometric properties, such as color, absorbance, intensity, and photo-luminescence, in specific locations of organic, inorganic, and biological samples located in analyte receptacles, e.g., cuvettes. For example, photometers are commonly utilized in biological research to test specimens for various chemicals, hormones, and enzymes. It is well known in the art that proper alignment of the container holding the analyte samples and the light beam of the photometer is necessary to perform many photometric measurements. Similarly, to perform image analysis, devices such as gel scanners and microscopes demand sample stages that provide consistent and accurate positioning of the analyte receptacle, e.g., a glass slide. In these applications, repeatable positioning of the analyte receptacle is also important for re-analysis of the image. Moreover, for imaging devices that utilize electrophoresis gels, alignment of these gels with respect to the electrophoreses axis is critical for consistent measurements.
Many photometric instruments make discrete measurements using multisite analyte receptacles called "microplates", which generally comprise rectangular structures made of glass or plastic, each having a multiplicity of wells (e.g. cylindrical wells) for holding analyte samples. A microplate allows preparation of a large number of test samples and may contain twenty four, forty eight, ninety six, or any other number of wells. Microplates are inexpensive, safe, sturdy, and convenient to handle. They are disposable, but can be cleaned easily and may be reused when necessary.
FIG. 1 illustrates one of the mechanisms currently available for positioning analyte receptacles, such as microplates, with respect to a measuring instrument. This mechanism comprises a carriage 100, slidingly mounted in an enclosure 102 that houses a measuring instrument, e.g., a photometer (not shown). A through rectangular opening 104, formed in carriage 100, accommodates an analyte receptacle 108. FIG. 2 shows, from a top view, an analyte receptacle 108 sitting in carriage 100. Analyte receptacle 108 contains a plurality of wells 109 for holding analyte samples and has reference planes 103, 105, 110, and 112. Carriage 100 contains a pair of compression springs 106 and 107 used to align analyte receptacle 108 in carriage 100. As illustrated in FIG. 2, when analyte receptacle 108 is inserted into rectangular opening 104 of carriage 100, the force of compression springs 106 and 107 directed against reference planes 105 and 103 respectively, aligns reference planes 110 and 112 of analyte receptacle 108 against the planar inner walls 114 and 116, respectively, of rectangular opening 104. In this position, a portion of the bottom of analyte receptacle 108 rests on a lip 101 which surrounds the rectangular opening 104.
The above-described apparatus, however, possesses several salient flaws. Specifically, to insert analyte receptacle 108 into carriage 100, the resistance of compression springs 106 and 107 must be overcome. As a result, analyte receptacle 108 suddenly snaps into position as the spring resistance is surmounted. The abrupt movement of analyte receptacle 108 may cause the contents of wells 109 to spill out, contaminating adjacent wells and making accurate measurements impossible.
For the above-mentioned reason, robotic insertion of analyte receptacle 108 into carriage 100 is difficult. A precisely-directed positioning force, capable of preventing sudden movements of analyte receptacle 108 and overcoming the resistance of springs 106 and 107 is required for this task. Most reasonably-priced robotic placement mechanisms possess a degree of error that makes them inadequate for reliably inserting analyte receptacle 108 into carriage 100.
Should the samples located in wells 109 become misaligned with respect to the measuring and/or viewing instrument, the instrument will not be able to accurately perform the required measurements and/or viewing tasks. The apparatus of FIG. 1 is deficient in its reliance on the control of the shape of analyte receptacle 108 for accurate alignment thereof with the scanning mechanism of the measuring instrument. Careful control of the shape of analyte receptacle requires a potentially expensive, precise manufacturing process. It is apparent from FIG. 2 that alignment of analyte receptacle 108 is achieved through aligning its reference planes 110 and 112 with planar inner walls 114 and 116. Since whole surfaces of inner walls 114 arid 116 are being used as reference planes, imperfections anywhere on the surfaces of reference planes 110 and 112 (e.g. a bump) will result in misalignment of analyte receptacle 108 with respect to the measuring instrument. Moreover, extraneous particles may become trapped between reference planes 110 and 112 of analyte receptacle 108 and planar inner walls 114 and 116 of rectangular opening 104, further hampering the alignment accuracy.
As each analyte sample decreases in size and as the number of analyte samples on a particular analyte receptacle increases, alignment of the samples relative to the measuring instrument becomes critical. Currently, silicon wafers find growing use as analyte receptacles for such procedures as drug discovery, where a large number of test sites is required. For example, when it is necessary to identify a specific protein sequence for binding with a certain type of receptor, a high density of samples in the analyte receptacle (that is, a large number of analyte samples on a particular analyte receptacle) is needed to expose the receptor to as many different permutations of proteins as possible. Therefore, in this example, the samples to be assayed are located on the surface of a silicon wafer in a multitude of discreet microscopic locations, with each discreet microscopic location containing a single microscopic sample. The centers of these microscopic samples are generally positioned approximately 50 microns away from each other, thus allowing one to place about 40,000 assays in an area of one square centimeter. Because of the small size and close spacing of the analyte samples, the wafer must be precisely aligned with respect to the measuring apparatus, thus allowing the measuring apparatus to make error-free measurements of the samples.